Spring steels are used in automobiles or machines in general. When a spring steel is used for an automobile suspension spring, for example, the spring steel must have high fatigue strength. Recently, there has been a need for automobiles having reduced weight and higher power output for improved fuel economy. Accordingly, spring steels that are used for engines or suspensions are required to have even higher fatigue strength.
Steel products may contain oxide inclusions typified by alumina. Coarse oxide inclusions decrease fatigue strength.
The alumina forms when the molten steel is deoxidized in the refining step. Ladles or the like often contain alumina refractory materials. For this reason, alumina may form in the molten steel not only in the case of Al deoxidation but also when deoxidation is carried out with an element other than Al (e.g., Si or Mn). Alumina in the molten steel tends to agglomerate and form clusters. In other words, alumina tends to be coarse.
Techniques for refining oxide inclusions typified by alumina are disclosed in Japanese Patent Application Publication No. 05-311225 (Patent Literature 1), Japanese Patent Application Publication No. 2009-263704 (Patent Literature 2), Japanese Patent Application Publication No. 09-263820 (Patent Literature 3), and Japanese Patent Application Publication No. 11-279695 (Patent Literature 4).
Patent Literature 1 discloses the following. A Mg alloy is added to the molten steel. As a result, the alumina is reduced and instead spinel (MgO.Al2O3) or MgO is formed. Consequently, coarsening of the alumina due to agglomeration of the alumina is inhibited.
However, the production method of Patent Literature 1 poses the possibility of nozzle clogging in a continuous casting machine. In such a case, coarse inclusions are more likely to become entrapped in the molten steel. This results in reduced fatigue strength of the steel.
Patent Literature 2 discloses the following. The average chemical composition of SiO2—Al2O3—CaO oxides at a longitudinal cross-section of the steel wire rod is controlled to be SiO2: 30 to 60%, Al2O3: 1 to 30%, and CaO: 10 to 50% so that the melting point of the oxides is not more than 1400° C. Furthermore, 0.1 to 10% of B2O3 is included in the oxides. As a result, the oxide inclusions are finely dispersed.
However, although B2O3 is effective for the above oxides, it sometimes cannot inhibit alumina clustering sufficiently. In such a case, the fatigue strength decreases.
Patent Literature 3 discloses the following. In the method of producing an Al-killed steel, an alloy made of two or more selected from the group consisting of Ca, Mg, and rare earth metal (REM) and Al is added to the molten steel for deoxidation.
However, in some cases, addition of the above alloy to a spring steel does not cause refinement of oxide inclusions. In such cases, the fatigue strength of the spring steel decreases.
Patent Literature 4 discloses the following. The bearing steel wire rod includes equal to or less than 0.010% of REM (0.003% in the example) so that inclusions can be spheroidized.
However, in some cases, addition of the above content of REM to a spring steel does not cause refinement of oxide inclusions. In such cases, the fatigue strength of the spring steel decreases.
Furthermore, suspension springs have the role of absorbing vibrations of the vehicle body caused by irregularities of the road surface on which it is traveling. Accordingly, suspension springs must have not only fatigue strength but also high toughness.
Methods for producing a spring include hot forming and cold forming. In cold forming, coiling is performed by cold operation to produce springs. Accordingly, spring steels must have high ductility for cold operation.